The present invention relates to a Roots blower with improved clearance between its rotors and improved volumetric efficiency.
As is known, Roots type blowers are of simple construction and are relatively trouble free. For these reasons, Roots blowers are widely used, such as a supercharger of an internal-combustion engine in which secondary pressure is relatively low or an air blower for various industrial equipments. For these blowers, various shapes or profiles of their blowers are used, for example, a cycloidal shape, an involute shape or an envelope shape.
A typical known Roots blower is of the straight two-lobe rotary type. Its housing has a suction port at its inlet side and a delivery port at its outlet side. Between the suction and delivery ports is communicatively interposed a rotor chamber. There is a 90-degree phase difference between the two rotors in the rotor chamber. The rotors are fixedly supported respectively on parallel rotor shafts. The rotor shafts lie in a plane perpendicular to a line including the centers of the suction and delivery ports. The ends of these rotor shafts are rotatably supported by a wall of the housing between the rotor chamber and a gear chamber in the housing.
The other ends of the rotor shafts rotatably supported by the wall extend into the gear chamber through the wall. The shaft ends fixedly support two gears in the gear chamber, respectively, which are meshed with each other.
Thus, the rotor shafts and their rotor synchronously engages and rotates in an opposite direction at the same speed. As a result, a fluid such as air flows from the suction port to the delivery port.
As the fluid is air in the case where this type of Roots blower is used for a compressor of an internal combustion engine, the two rotors must be rotated without lubricating oil. Furthermore, mechanical interference may occur between various parts because of high rotational speed. For example, interference may occasionally occur between the two rotors. Another possibility is the interference between the two rotors and the wall. Therefore, an appropriate clearance must be provided therebetween in order to avoid such interference.
However, if the clearance is large, the air being pumped leaks from the clearance during the rotation of the rotors. Consequently there arises the problem of a drop in the volumetric efficiency of the blower.
Accordingly, it is necessary to prevent the volumetric efficiency from dropping. For this purpose, it is necessary to reduce the clearances by considering the following factors.
1. Backlash of the two gears for synchronizing the phase of the rotor. PA1 2. Assembly error for synchronizing the phase of the two rotors. PA1 3. Fabrication error of the distance between centers of the two rotors. PA1 4. Fabrication error of the profile of the rotor. PA1 5. Thermal expansion of the rotor due to heat of compressed air by the rotation of the rotors.
The clearance therefore must be as small as possible by considering the factors written above, and thus it is required to prevent volumetric efficiency from decreasing. Most important factor of the above is the phase error between the two rotors. As for the other factors, it is possible to reduce the clearances by improving the fabrication precision.
The clearance between the rotors is provided for prevention of interference therebetween. The clearance is generally formed by providing a specific relief or an offset with respect to the profile of the rotor. That is, the relief or the offset quantity is provided with a combination of epicycloidal and hypocycloidal curves of the rotor profile defined.
A technique for reducing the clearance between the rotors to a minimum is disclosed in Japanese Patent Laid-Open Publn. No. 75793/1985. According to this prior art, a secondary relief quantity or the offset quantity of the rotor basic profile is determined by an angle between a normal line at a point on an outer periphery of the rotor basic profile and a line connecting the two rotor centers.
More specifically, a curve is obtained by relieving a minimum clearance (i.e. a primary relief quantity) necessary for permitting rotation of the rotor without contacting each other from the original profile curve of the rotor. That is, the basic curve is obtained by reducing a specific quantity in the normal direction from the profile curve of the rotor. Then, secondary relief quantity is determined with a function of increasing or decreasing the profile in correspondence with the above mentioned angle with respect to the basic curve. Then, a fabrication or assembly error is added to the above mentioned primary relief quantity. In this manner, the profile of each rotor is finished.
However, in the above described prior art, it has been necessary to determine the finally finished profile by adding the secondary relief quantity or offset to the primary ones. For this reason, the method is indirect, and errors are easily increased during fabrication of the rotors. Thus, there has been a limit inherently to the reduction of the clearance between the rotors while interference of the rotors is being prevented.
Therefore, there has remained the problem of attaining a large improvement in the volumetric efficiency of the Roots blower. For this purpose, setting of the clearance between the rotors must be made even more accurate. In order to achieve this accuracy, high-precision fabrication of the rotors is imperative.